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PlatyhelmintesVisible as paired anterior eye spots that detect light levels rather than images chemosensory auricles on each side of the head and an anterior brain that coordinates sensory input from the eye spots and auriclesBilateralsymmetryMore advanced than the preceding radial symmetry or biradial symmetryAnterior and posterior ends left and right sides as well as dorsal and ventral surfacesAdvantages to being able to sense where youre going and there is increasing complexity of the nervous system and a concentration of sensory structures at the anterior end of the animal Its a process referred to as cephalization a first with the flatworms but with a difference In other cephalized animals the mouth is located at the anterior end but thats not the case in flatworms and their mouth is located on the ventral surface The mouth connects with an incomplete gut that branches to varying degrees throughout the body or disappears completely in some of the parasitic speciesTriplobastacoelomatesThe Platyhelminthes are the first triploblastic animals and have a third tissue layer the mesoderm added between the ectoderm and endoderm that cnidarians had Can now have muscles formed from mesodermFlatworms mesoderm is solid without a cavity a coelom inside Its why they are referred as acoelomateOnly way a solid animal can maintain an adequate surfacetovolume ratio is to be as flat as possible Solid animals have to depend on diffusion to move oxygen nutrients and metabolic wastesDigestive system is incomplete but branches throughout the body making sure that every cell is close to a supply of nutrients The guts content are mixed by the lined cilia and the animals movementThe fluids in the gut and the surrounding environment are important for gas exchange and removal of protonephridia Osmoregulatoryexcretory structure for osmoregulation and are most obvious in freshwater species which are hyperosmotic to the water around them Protonephridia are also called flame
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